The present invention relates to a fixing control method and apparatus in an electrophotographic system and, more particularly, to a method and apparatus for fixing by using an induction heating (to be referred to IH hereinafter) coil.
Apparatuses using conventional IH coils are disclosed in, e.g., the following references.
(a) Japanese Patent Laid-Open No. 2000-215976
An IH coil is composed of a main heating coil and an auxiliary heating coil. The amount of heat generated in an end portion of a fixing roller is varied by arbitrarily changing the amount of heat generated by the auxiliary heating coil, thereby uniformizing the temperature.
(b) Japanese Patent Laid-Open No. 2000-206813
A plurality of excitation coils are formed to control the electric power of a heating unit for heating the two end portions of a fixing roller, thereby uniformizing the heat distribution of the fixing roller.
In each of these techniques disclosed in references (a) and (b), temperature variations and generated heat variations of a fixing roller of a fixing device using an IH coil are controlled by varying the electric power supplied to heat the two end portions of the fixing roller. In this manner, the heat distribution of the fixing roller is held constant.
Unfortunately, in these systems in which the supply of power is partially controlled, it is difficult to set the length by which the fixing roller is divided. In particular, domestic paper sheets have unique standards (B sizes), so a number of sheet sizes are present. In addition, even sheets of the same size can be used in landscape and portrait positions. Therefore, in a system in which a fixing coil is divided into, e.g., at most three portions, the heat distribution of a fixing roller cannot be uniformized.
Also, when a fixing coil is divided into a plurality of portions, IH control circuits equal in number to the divided portions are necessary. This complicates the circuit and increases the cost.
(c) Japanese Patent Laid-Open No. 11-30928
A fixing device disclosed in this reference can fix images on thick paper sheets by correcting a fixing control temperature even when the environmental temperature changes. That is, this technique can meet changes in the environment and the types of sheets by changing the fixing control temperature.
After the fixing control temperature is changed, however, a fixing roller takes a long time to reach that temperature. Accordingly, this control largely prolongs the first print time before printing of the first sheet is started.
As described above, the method which divides an IH coil cannot uniformize the heat generation of a fixing roller in accordance well with the sizes of various sheets. Also, dividing an IH coil complicates the circuit and increases the cost. The method which corrects the fixing control temperature undesirably prolongs the first print time.
The present invention has been made in consideration of the above situation, and has as its object to provide a fixing control method and apparatus capable of uniformizing the heat generation of a fixing roller without increasing the first print time.
A fixing control apparatus of the present invention is an apparatus for performing a fixing process by using a heat roller heated by an induction heating coil, characterized by comprising a first temperature sensor positioned, around the heat roller, within the range of xc2x145xc2x0 from a portion where maximum heat is generated by the induction heating coil, a second temperature sensor positioned, around the heat roller, within the range of xc2x145xc2x0 from a portion where minimum heat is generated by the induction heating coil, and a central processing unit for performing switching control of a fixing control temperature concerning the temperature of the heat roller on the basis of temperatures sensed by the first and second temperature sensors.
In this apparatus, when power supply to the induction heating coil is started, the central processing unit can change at least one of the fixing control temperature, a pre-run start temperature at which pre-run is started, and a ready display temperature pertaining to temperature display in a ready state, on the basis of the temperatures sensed by the first and second temperature sensors.
Alternatively, when power supply to the induction heating coil is started, the central processing unit can lower the fixing control temperature and the pre-run start temperature, as the temperature sensed by the first or second temperature sensor rises.
The fixing control temperature can be changed such that the temperatures sensed by the first and second temperature sensors fall within a predetermined printing permissible temperature range in a ready state, the fixing control apparatus can further comprise an environmental temperature sensor for sensing an environmental temperature, and the central processing unit can change the fixing control temperature on the basis of the environmental temperature sensed by the environmental temperature sensor.
At the start of printing, the central processing unit can change the fixing control temperature on the basis of the temperatures sensed by the first and second temperature sensors, the fixing control apparatus can further comprise an environmental temperature sensor for sensing an environmental temperature, and, on the basis of the environmental temperature sensed by the environmental temperature sensor, the central processing unit can change, in accordance with a sheet to be printed, the fixing control temperature and a threshold value for determining whether an environment is a normal environment or a low-temperature environment on the basis of the environmental temperature sensed by the environmental temperature sensor.
Alternatively, at the start of printing, the central processing unit can check whether the temperatures sensed by the first and second temperature sensors fall within a predetermined printing permissible temperature range, permit printing if the temperatures fall within the range, and perform pre-run if not, the fixing control apparatus can further comprise an environmental temperature sensor for sensing an environmental temperature, and, on the basis of the environmental temperature sensed by the environmental temperature sensor, the central processing unit can change the printing permissible temperature range in accordance with a sheet to be printed.
During printing, the central processing unit can change the fixing control temperature for each sheet to be printed whenever a predetermined time elapses, on the basis of a result of comparison of the temperatures sensed by the first and second temperature sensors with a lower-limiting threshold value of a printing permissible range, the fixing control apparatus can further comprise an environmental temperature sensor for sensing an environmental temperature, and, on the basis of the environmental temperature sensed by the environmental temperature sensor, the central processing unit can change the lower-limiting threshold value.
In an operation mode in which pre-run continues for not less than a predetermined time, the central processing unit can lower the fixing control temperature or stop power supply to the induction heating coil.
A fixing control method of the present invention is characterized in that switching control of a fixing control temperature concerning the temperature of a heat roller is performed on the basis of a temperature sensed by a first temperature sensor positioned, around the heat roller, within the range of xc2x145xc2x0 from a portion where maximum heat is generated by an induction heating coil, and a temperature sensed by a second temperature sensor positioned, around the heat roller, within the range of xc2x145xc2x0 from a portion where minimum heat is generated by the induction heating coil.